The present invention relates to a method and apparatus for directional radio communication in which signals between a first station and a second station may be transmitted only in certain directions. In particular, but not exclusively, the present invention is applicable to cellular communication networks using space division multiple access (SDMA).
With currently implemented cellular communication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by that base transceiver station. However, space division multiple access (SDMA) systems have now been proposed. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in the beam direction from which a signal from the mobile station is received. SDMA systems may also permit the base transceiver station to determine the direction from which signals from the mobile station are received.
SDMA systems may allow a number of advantages over existing systems to be achieved. In particular, as the beam which is transmitted by the BTS may only be transmitted in a particular direction and accordingly may be relatively narrow, the power of the transceiver can be concentrated into that narrow beam. Likewise, the signal transmitted to the BTS by, for example, a MS will be received by the BTS only in a limited number of beam directions. It is believed that this results in a better signal to noise ratio with both the signals transmitted from the base transceiver station and the signals received by the base transceiver station. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver station can be achieved. Furthermore, in the transmitting direction, the directionality of the BTS allows energy to be concentrated into a narrow beam so that the signal transmitted by the BTS can reach far away located mobile stations with lower power levels than required by a conventional BTS. This may allow mobile stations to operate successfully at greater distances from the base transceiver station which in turn means that the size of each cell or cell sector of the cellular network can be increased. As a consequence of the larger cell size, the number of base stations which are required can also be reduced leading to lower network costs. SDMA systems generally require a number of antenna elements in order to achieve the required plurality of different beam directions in which signals can be transmitted and received. The provision of a plurality of antenna elements increases the sensitivity of the BTS to received signals. This means that larger cell sizes do not adversely affect the reception of signals by the BTS from mobile stations.
SDMA systems may also increase the capacity of the system, that is the number of mobile stations which can be simultaneously supported by the system is increased. This is due to the directional nature of the communication which means that the BTS will pick up less interference from mobile stations in other cells using the same frequency. The BTS will generate less interference to other mobile stations in other cells using the same frequency when communicating with a given MS in the associated cell.
Ultimately, it is believed that SDMA systems will allow the same frequency to be used simultaneously to transmit to two or even more different mobile stations which are arranged at different locations within the same cell. This can lead to a significant increase in the amount of traffic which can be carried by cellular networks.
SDMA systems can be implemented in analogue and digital cellular networks and may be incorporated in the various existing standards such as GSM, DCS 1800, TACS, AMPS and NMT as well as he proposed next generation standards such as, for example, UMTS (Universal Mobile Telecommunications System). SDMA systems can also be used in conjunction with other existing multiple access techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA) techniques.
One problem with SDMA systems is that the direction in which signals should be transmitted to a mobile station needs to be determined. In certain circumstances, a relatively narrow beam will be used to send a signal from a base transceiver station to a mobile station. Therefore, the direction of that mobile station needs to be assessed reasonably accurately. As is known, a signal from a mobile station will generally follow several paths to the ETS. Those plurality of paths are generally referred to as multipaths. A given signal which is transmitted by the mobile station may then be received by the base transceiver station from more than one direction due to these multipath effects.
In general, the decision as to the beam direction which is to be used by the BTS in order to transmit a signal to a mobile station is based on information corresponding to the data burst previously received by the BTS from the given MS. As the decision is based on information received corresponding to only one burst, problems may occur if, for example, the data burst transmitted by the mobile station is superimposed with strong interference.
An additional problem is that the direction in which a signal is to be transmitted by the BTS to the mobile station is determined on the basis of the signals received by the BTS from the mobile station. However, the frequencies of the signals transmitted from the mobile station to the BTS are different from the frequencies used for the signals transmitted by the BTS to the mobile station. The difference in the frequencies used in the uplink and downlink signals means that the behaviour of the channel in the uplink direction may be different from the behaviour of the channel in the downlink direction. Thus the optimum direction determined for the uplink signals will not always be the optimum direction for the downlink signals. In other words, the statistical behaviour of the channel in the up-link and down-link directions are different. This also means that it is not possible to have a fast and effective (burst-by-burst basis) power control as there is in general no fast (burst-by-burst) feedback from the MS.
It has been proposed by the present inventor that a signal from a base transceiver station to a mobile station be sent in two adjacent beam directions. This means that the base station generates two or more separate beams. When those beams are adjacent one another, they should overlap. By allowing the beams to overlap the whole cell or cell sector can be covered. However, due to differences in the effective path length travelled by the signals to a beam former of the base transceiver station, adjacent beams may have an effective phase difference therebetween. Depending on the value of this phase difference, a null region may occur in the overlapping region of two adjacent beams. Any mobile station in that null region would be unable to receive signals from the base transceiver station. Another problem arises when more than one beam direction is selected. If the power of the beams is set to be equal, this can undesirably give rise to increased interference.
It is therefore an aim of certain embodiments of the present invention to address some of the problems mentioned hereinbefore.
According to a first aspect of the present invention, there is provided a method of directional radio communication between a first station and a second station, said method comprising the steps of defining at the first station a plurality of beam directions for transmitting signals to said second station, each of said beam directions being selectable; selecting a plurality of beam directions at said first station in which a signal is to be transmitted from said first station to said second station; consecutively transmitting said signal in said plurality of beam directions, whereby the power level of the signal transmitted in each of said selected plurality of beam directions is individually selectable.
For the purpose of this document, the term signal should be broadly interpreted. For example, a burst of data in a GSM system may constitute xe2x80x9ca signalxe2x80x9d. Alternatively, a plurality of bursts of data in a GSM system may constitute xe2x80x9ca signalxe2x80x9d.
It has been recognized that better results can be achieved, where two or more beams are selected if the power of each of the selected beams is individually selectable.
By also altering the power of each beam, a more flexible shaping of the beam pattern can be obtained thus reducing the possible interference to non-desired stations. This in turn means that it might be possible to improve the system capacity. Thus, if the power of each beam is individually selectable, it is possible to flexibly alter the shape of the beam pattern.
Preferably, first and second sequential signals are to be transmitted to said second station, said method further comprising the steps of:
altering the phase of the first and second signals to be transmitted in at least one of said selected beam directions;
whereby the phase of the first signal transmitted in at least two of the beam directions differs, the phase of the second signal transmitted in said at least two beam directions differs and the phase difference of the first signal transmitted in said at least two beam directions is different from the phase difference of said second signal transmitted in said at least two beam directions.
By ensuring that the phase difference of the first signal transmitted in the two beam directions is different from the phase difference of the second signal transmitted in the two beam directions, the problems caused by null regions can be considerably reduced. In particular, even if a null region occurs for one signal, it is unlikely to be present for the second signal in that the phase difference between adjacent beams is changed for the next signal. It should be appreciated that the phase of the signals transmitted in one beam direction may be unaltered but the phase of the signals in another beam direction may be altered. Alternatively, the phase of the signals may be altered for all of the selected beam directions.
Preferably, at least two of the beam directions are adjacent. It is preferred that the first station be arranged to transmit a multiplicity of consecutive signals to the second station and that the phase of each signal be altered such that the phase difference between each consecutive signal transmitted in at least two of the beam directions is different for consecutive signals. Thus, when the response of the first station is averaged over time, the probability that null regions occur can be reduced.
Preferably, the phase of the consecutive signals is randomly altered. The random alteration of the phase is able to create a spatial modulation of the resulting beam pattern, particularly when considered over a relatively large number of consecutive signals. However, it is also possible that the phase of the consecutive signals can be altered in accordance with a predetermined pattern. It is preferred that this predetermined pattern allows the desired spatial modulation of the beam pattern to be achieved.
Preferably, method also includes the steps of receiving at said first station a plurality of signals from said second station, said signals being receivable from a plurality of beam directions; determining for at least one of said beam directions a value of a parameter of at least one signal received from said second station in said at least one beam direction; looking up in a look-up table a power value corresponding to the determined value; and transmitting a signal to the second station in said at least one beam direction, the power of the signal transmitted in said at least one beam direction being determined by the power value looked up in the look-up table. Preferably the received signals are consecutive
According to a second aspect of the present invention, there is provided a method of directional radio communication between a first and a second station, said method comprising the steps of receiving at said first station a plurality of consecutive signals from said second station, said signals being receivable from a plurality of beam directions; determining for at least one of said beam directions a value of a parameter of at least one signal received from said second station in said at least one beam direction; looking up in a look-up table a power value corresponding to the determined value; and transmitting a second signal to said second station in said at least one beam direction, the power of the signal in said at least one beam direction being determined by the power value looked up in the look-up table.
The use of a look-up table is particularly advantageous in that it provides a simple way of determining the power level of a signal to be transmitted to the second station.
Preferably, a mean value of the parameter for a plurality of signals is determined and the power value corresponding to the determined mean value is looked up in a look-up table. This parameter may be the energy of the signals. Alternatively, the parameter can be one or more of the following parameters: instantaneous energy of a signal; type of radio environment; or distance between the first and second stations.
There are advantages in being able to determine the power of a signal to be transmitted to the second station based on an average of a number of preceding signals received from that second station. This is because although for a single signal the behaviour of a channel between the first station and the second station is not the same as a channel between the second station and the first station, on average, the behaviour of the channels in both directions will be similar. By taking into account the signals received over a period of time, it can be assumed that the channel between the first station and the second station will on average be similar to that between the second station and the first station. It will be noted that this method of determining the power will generally be effective on average and not necessarily effective for every single signal.
The mean energy determined in said determining step may be quantized and the quantized mean energy be associated by the look-up table with a corresponding power value. This makes the look-up table easier to achieve in practice.
The power value represents the power of the signal to be transmitted in the beam direction or alternatively may represent a control value for controlling the setting of a power level for a signal in a given beam direction. For example, if an amplifier were present, the control value could be used to control the amplifier to provide the required signal amplification.
The values in the look-up table may be altered in accordance with a parameter of the first station and/or the second station. Alternatively, the values of the look-up table may be fixed and not be alterable. Where the values of the look-up table are altered, they may be altered in accordance with the power measuring reports received from the second station. The values for the look-up table are preferably determined based on one or more of the following:
transmission power used by said second station;
distance between said first and second station;
the mean energy of the signals received from the second station in a given beam direction;
distance between the first and second station;
the radio environment;
the validity of a known attenuation law in a channel defined between the first and second stations.
The number of signals used to calculate the mean value may be varied depending for example on the degree of correlation between the channel between the first station and the second station and the channel between the second station and the first station. The number of signals used to calculate the mean value may be dependent on the signal quality reports received from the second station.
The energy of each of the signals received in the given beam direction is preferably determined from the channel impulse response. This calculation is generally carried out by most communication networks and thus can be utilised by embodiments of the present invention.
Preferably, the first station is a base transceiver station in a cellular network. The second station is preferably a mobile station in said cellular network. Preferably, the signals are burst, and the phase is altered on a burst-by-burst basis.
As will be appreciated, aspects of the first invention can be used with aspects of the second invention and vice-versa.
According to a third aspect of the present invention, there is provided a first station for directional radio communication with the second station, said apparatus comprising transmitter means for transmitting a signal in a plurality of beam directions, each of said beam directions being selectable; selection means for selecting a plurality of beam directions in which a signal is to be transmitted from the first station to the second station; and control means for controlling said transmitter means, wherein said control means is arranged to individually control the power level of the signal transmitted in each of said selected beam directions.
Preferably, said station is arranged to transmit first and second signals to said second station, said first station comprising phase altering means for altering the phase of the first and second signals to be sent in at least one of the selected beam directions; and wherein said control means is arranged to control the transmitter means to transmit the first and second signals in said plurality of beam directions, whereby the phase of the first signal in at least two of the beam directions differs, the phase of the second signal in said at least two of the beam directions differs and the phase difference of said first signal in said at least two beam directions is different from the phase difference of the second signal in said at least two beam directions.
Preferably, the means for altering the phase comprises a phase modulator. The phase modulator may be arranged between the input of a beam former of said transmitting means and a signal processor of the first station.
According to a fourth aspect of the present invention, there is provided a first station for directional radio communication with a second mobile station, said apparatus comprising receiving means for receiving a plurality of consecutive signals transmitted by said second station, said signals being receivable from a plurality of different beam directions; determining means for determining for at least one of the beam directions the value of a parameter of at least one of signals received from the second station in said at least one beam direction; a look-up table for providing power values corresponding to the determined value; and transmitting means for transmitting a second signal to the second station in said at least one beam direction, the power level of the signal being determined by the power value looked-up in the look-up table.
The determining means may be arranged to determine a mean value of the parameter for a plurality of signals. The parameter may be the energy of the signal.
Preferably, the transmitter means of either the third or the fourth aspect comprise an antenna array which is arranged to provide a plurality of signal beams in a plurality of different directions.